2001 Chev HD2500, 265K miles, (Eaton) 11.5" ring gear:
Something I've noticed for quite some time is that after traveling a long distance (100+ miles), I feel and hear an audible "clunk" after moving in reverse- meaning, if I put the truck into reverse and back up a few yards, eventually, I'll hear/feel this noise. I run the Mobil 1 70W-90 synthetic lube and have been for years. Rarely do I carry heavy loads and my towing usually consists of a john boat for duck hunting or an ATV/trailer. I travel 250+ miles one way 2X weekly for work. When I've pulled the cover and changed lube in the past, I've never found any debris in the lube and there are no teeth missing that I can see from the R/P or the side/spider gears. Is there a possibility that the clutch disks are binding and finally "let loose" when backing up? I noticed this condition long ago (before I had 50K miles on the truck). I hear no bearing noise coming from the rear end or from the wheels.

I think the locking diff would be my first guess also, but it could also be a worn universal joint, and even though it sounds like it's coming from the rear check all of the u-joints....sound can travel the length of the truck through the drivetrain.

I think that Mobile 1 is really good stuff so I don't think it's failing unless the diff is under-filled. The easy thing to do is check the fluid level, then maybe try some additive in the rear diff. Also, if you have close to 50K on the current fluid I would change it.

Thanks for the reply. U-joints are good. Have I got a problem with the gov-lock, maybe partially engaging? Or the side disks are binging? Hard to imagine either, but.... I noticed this condition shortly after the truck was newer, somewhere in 2002! I think DmaxMaverick once told me not to include any additives; not needed and could do damage. I used to run the Mobil 90W-140, but that was too heavy for my application. I switched to the 70W-90, but I agree it's time to drain and refill with fresh. Hell, for that matter, a new set of bearings and spider shims may not be bad either.
Thanks again.

Unless something is "broken" with the Gov-Loc (I mean really broken), it isn't a factor. The assembly only ever does anything when there is a wheel speed differential (during a rolling turn, or traction loss). Otherwise, the entire carrier, including the locker, rotates as a single mass. If you do, in fact, have a clunk caused by the carrier, the problem will be more traditional, which includes gear lash or bearing failures. Shifting in/out of a gear range will not identify any issues with the Gov-Loc, short of the need for a complete diff rebuild (you would have very significant issues at almost any other time than that). Drain/fill the lube and see what comes out and what's on the plug (magnetic). Any larger chunks of metal could mean a failure, but if all you get is a bit of powdery sludge, it's normal.

The most common issues that can cause a clunk are slip-yoke (where it splines into the tranny/TC), brakes or U-joint related. I suggest checking the brakes first (adjuster pins, caliper/pad movement), then the yoke. If the yoke is under-lubricated, the slight spring wrap occurring during shifts can cause a clunk. A good packing with high pressure moly or lithium grease will cure this. A 4x4 will have more points of connection in the drivetrain, and can also be exaggerated, especially with more than one condition at a one time (or the TC could be the cause, in itself). A two-piece drive shaft can also increase the gremlin count (bad isolator and/or carrier bearing). Eliminate the common issues first. If no joy, start at one end and work your way to the other. Which end really doesn't matter, at this point.